Roundcube is a web-based IMAP email client. Roundcube's most prominent feature is the pervasive use of Ajax technology. After about two years of development, the first stable release of Roundcube was announced in early 2008.

Roundcube is written in PHP and can be employed in conjunction with a LAMP "stack", or any other operating systems that support PHP are supported as well. The web server needs access to the IMAP server hosting the email and to an SMTP server to be able to send mails.

With version 0.3 Roundcube introduced a plug-inAPI which allows non-standard features to be added without the need to modify the source code. A variety of plug-ins are available from the Plugin Repository.